The present invention relates generally to the field of harmonic frequency generation/frequency multiplication, and more particularly to oscillator/doublers for generating the second harmonic frequency.
Due to the increased utilization of the millimeter-wave frequency range, there is a significant need for millimeter-wave generation devices for generating RF power at these frequencies. There are a variety of advantages such as ease of fabrication and packaging which accrue if the aforesaid millimeter-wave generating device can be formed in the solid-state domain. In this regard, there are already a number of transistors which are capable of fundamental frequency oscillation up through at least 40 GHz. An example of such a transistor is the GaAs FET utilizing sub-half-micron gate lengths. Moreover, the millimeter frequency generation range for such transistors can be appreciably extended even further by exploiting the nonlinear properties of the device in order to obtain harmonics of the fundamental, i.e. frequency multiplication. Such frequency multiplication is used to generate microwave and millimeter wave power at frequencies at which it is not practical to achieve fundamental frequency oscillation due to a lack of transistor gain. It has been found that the use of the frequency multiplication technique is especially advantageous for obtaining power at the second harmonic frequency because limitations imposed by parasitic effects common at higher frequencies are avoided.
The present invention sets forth a solid-state design for exploiting the frequency multiplication technique. The basic circuit to be considered in the present design consists of a transistor and a coupling network. Such a transistorcoupling network combination provides oscillation at the fundamental frequency f.sub.o as well as a significant signal power component at the second harmonic frequency at 2f.sub.o. Thus, the circuit functions both as a fundamental frequency oscillator and a harmonic frequency generator. Signal power is then extracted at the harmonic frequency only.
In order to achieve the maximum power conversion from DC to RF, optimum matching conditions are required for the input and the output of the transistor. A number of references have been directed to the problem of achieving maximum efficiency of power generation at the second harmonic frequency. By way of example, the paper "Performance and Design of Microwave FET Harmonic Generators" by M. S. Gupta, R. W. Laton, and T. T. Lee, IEEE Transactions On Microwave Theory And Techniques, Vol. MTT-29, No. 3, March 1981, sets forth a design for testing the frequency doubling characteristic of a GaAs MESFET with the input and output frequencies being 4 and 8 GHz, respectively. The input and output ports of the FET are matched and tuned at their respective frequencies by microstrip stubs. In addition, external coaxial stub tuners are used to optimize the doubler performance by varying the impedances presented to the device at various harmonic frequencies. The reference "Harmonic Load Pull" by R. B. Stancliff and D. D. Poulin, 1979 IEEE MTT-s International Microwave Symposium Digest, pp. 185-187, April 1979, sets forth another technique for independently tuning an oscillator signal and the second harmonic therefrom. In one embodiment, the tuning at the fundamental and the second harmonic is achieved by a slide screw tuner. However, neither of the above-cited references achieved maximum DC to RF power conversion efficiency.